code: def _compose_overlays(self):
      B = cache = None
      for overlay in self._overlays.values():
          A = overlay.to_colors()
          if B is None:
              B = A
          else:
              cache = B
              B = self._compute_over(cache, A)
      return B, cache
  
file_path: mne-python/mne/viz/_3d_overlay.py
imports: from collections import OrderedDict
  import numpy
  from ..utils import logger
label_desc: Use image node_id 4 for calling the _LayeredMesh obj's underlying member method code with example usage: obj._compose_overlays() and returns: B,  cache